The present invention relates to a wire bonding apparatus and a method for the same, and more particularly, to a wire bonding method and apparatus that enables mounting of a chip to a thin package even in cases when an electrode of a chip that includes a semiconductor device is separated from the edge of the chip and is positioned on the inside.
In wire bonding apparatus, it is generally required to change the shape of the loop of the wire in accordance with the electrodes on the chip.
FIGS. 1A and 1B show differences in the shape of the wire loop for the two different cases where a value B there distance from the bonding point of a chip electrode 3 to the end surface of a chip 2 is B1&lt;B2 when loop lengths L1 are both the same. Moreover, in the figure, numeral 1 is an island of a lead frame, 4 is an inner lead of the lead frame, 5 is a lead frame electrode on the inner lead 4, and 6 is a bonding wire.
Many chip electrodes are formed along the edge of the chip. Even if this electrode 5 are positioned in the middle portion of the chip, if it is possible for wire bonding to still be performed, then there will be for loss restrictions when there is the design of the semiconductor device formed in the chip. Then, the wires can be made slightly longer and connection made between the chip electrode 3 and the frame electrode 5. This is one requirement of recent design.
However, in addition to this requirement, there is also the requirement to have thinner semiconductor packages for mounting to chips. In a semiconductor device that has the chip electrode 3 formed in the middle of the chip, the wires that are slightly longer and loop-shaped have a height of about 200-259 .mu.m from the surface of the chip. The thickness of the package that has this chip and lead frame encapsulated by resin must be held to within 1mm overall because of the requirement for thinness. Because of this, the thickness of the resin from the surface of the chip to the outer surface of the package must be held down to 300 .mu.m or less but the state of the bending of the loop formed by the wire, and scattering in the resin when there is encapsulation results in the possibility that the loop may protrude from the surface of the package in some cases.
Namely, as shown in FIG. 1B, when the shape of the loop wire shown in FIG. 1A is applied, then as shown in FIG. 1A for the case of FIG. 1B, a chip corner portion 2a protrudes inside the bonding cavity between the chip electrode 3 and the lead frame electrode 5 and so this wire 6 therefore either comes to the vicinity of or comes into contact with the chip corner portion 2a, and the danger of shorting increases.
Because of this, for the case shown in FIG. 1B, the wire 6 is extended horizontally upwards from the chip electrode 3 and towards the chip corner portion 2a to form a straight portion so that the distance between the chip corner portion 2a the wire 6 is maintained, and the extended end of this straight portion is bent downwards to extend to the frame electrode 5.
Of course, when there is surplus in the upwards direction of the chip 2, the horizontal portion described above is bent, so there is a smooth curve instead of the straight portion described above.
In this manner, by changing the shape of the wire in accordance with the position of the electrode on the chip, it is possible to prevent shorting between the chip 2 and the wire 6.
The following is a description of the process of connecting the wire to the frame electrode and the chip electrode while changing the shape of the loop. The wire is a gold (Au) wire, and is passed by a jig called a capillary that tightens the wire according to necessity by using air inside its hollow tube-shaped cavity to pull the wire tight. The wire protrude slightly from the distal end of the capillary and when a high voltage is applied, the end of the wire melts to form a spherical shape. After this, the wire is pulled into the capillary by air suction and the spherical portion of the wire is positioned at the distal end. A chip electrode is formed from aluminum (Al) beforehand at a required position on the chip 2, and the chip that is placed on the lead frame is heat along with the frame to a temperature of about 250.degree. C. The spherical distal end of the wire is then pressed against the aluminum electrode of the heater chip and the electrode and the wire are joined and when ultrasonic waves are applied to this joined portion, the aluminum and gold at this portion crystallize to form the chip electrode. After this, the tension in the wire is temporarily cancelled and the capillary is raised at a constant amount, the wire is fed out, and the capillary moves by a constant amount in the direction opposite that of the lead frame. This constant amount of movement is termed "reverse displacement". Moreover, the constant amount that the capillary is raised is termed the "reverse height". After movement through the reverse height and the reverse displacement, the air is once again removed from the capillary, tension is again applied to the wire, and an angle (of 90.degree. to 180.degree. ) if formed in the wire. After this, the capillary rises by an amount equivalent to the distance to the lead frame electrode, while feeding out the wire once again from the capillary. This amount is defined as the Z rise amount. The capillary moves by the amount of the Z-rise amount and then describes an arc as it drops in the direction for the lead frame electrode. The motion of the distal end of the capillary in this drop movement is defined as the Z-drop path. The capillary guides the wire through this Z-rise and Z-drop to the position for the formation of the lead frame electrode, and presses the wire against the surface of the lead frame. At this time, the frame is heated and to a certain temperature and so the wire is clamped over the capillary and tensioned so that the wire breaks from the portion that is connected to the frame. This is the end of the process of connecting a wire from one chip electrode to one lead frame electrode. After this, the wire bonding processes is repeated for the connection of as many electrodes as necessary.
However, it is possible to change the shape of the wire by changing the tracking parameters (reverse movement amount, reverse height, Z movement amount and Z drop track) that determine the motion track of the capillary of the bonding apparatus.
However, it is necessary to use a computer aided design (CAD) system to change these tracking parameters, and this work becomes complex since it is necessary to confirm the shape of the loop when each of the parameters are actually set. Therefore, the work efficiency for a single chip drops in accordance with the number of the values B. In particular, when a straight portion such as that described above is provided for the shape of the wire, much labor is necessary for the selection of the combination of the tracking parameters.
In the conventional wire bonding method as described above, the setting of the tracking parameters involves much work and so when a single chip has many different distances between the electrode and the chip end surface, that is, when the chips protrude by different amounts from the bonding position on the chip electrode, there is the problem that there is a drop in the work efficiency.